Zonguldak Basin Depositional History and Stratigraphy
The Zonguldak basin is a geologically complex region characterized by two primary periods of deposition: one beginning in the Paleozoic era and a second starting in the Cretaceous. Between these major cycles, the basin experienced isolated intervals of deposition during the Late Permian, Triassic, and Latest Jurassic. This sequence of sedimentary layers provides a detailed record of shifting environments, from deep marine shelves to coastal plains and volcanic activity.
Key Facts
- Primary Deposition Eras: Paleozoic and Cretaceous.
- Coal-Bearing Units: The Alacaagzi and Kozlu Formations are the primary sources of economic coal.
- Major Unconformity: A significant angular unconformity exists atop the Karadon Formation, spanning 46.5 Ma to 194 Ma.
- Cretaceous Driver: Subsidence caused by the formation of the Black Sea back arc drove late-stage deposition.
- Diverse Lithology: Deposits range from green shales and arkosic conglomerates to limestones and andesitic tuffites.
Paleozoic Deposition
The initial sedimentary history of the Zonguldak basin began in the Ordovician period. This phase is marked by a progression from deep-water deposits to shallower, higher-energy environments.
Early to Middle Paleozoic Sequences
Deposition commenced with the Soğuksu Formation (lower Ordovician), which reaches thicknesses of 700–1100 meters. It transitions from green shale and sandstone at the base to arkosic conglomerates (sandstones rich in feldspar) at the top. This is followed by the Aydos Formation, a 50–200 meter thick conglomerate of quartzitic sandstone.
During the upper Ordovician, Silurian, and lowermost Devonian, the Findikli Formation was deposited. Spanning 300–450 meters, its facies indicate a mixed siliciclastic-carbonate shelf environment that gradually became shallower over time.
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Transition to Shallow Marine and Coal-Bearing Strata
The Ferizli Formation overlies the Findikli, consisting of red, cross-bedded sandstones. These oolitic sandstones contain iron ore and signal a shift toward higher-energy, shallower depositional environments. As calcium carbonate enrichment increased, these sediments transitioned into the Yilanli Formation.
The Yilanli Formation (Visean age) marks the start of coal-related sequences. It consists of dolomitic limestone with calcareous black and gray shales, deposited in a shallow marine passive margin setting. This is followed by the Alacaagzi Formation, which accumulated over 1000 meters of sediment. This unit is economically vital due to its coal deposits, transitioning from black shales and silts to sands, shale-bearing coals, and conglomerates, suggesting coastal, lacustrine (lake), fluviatial (river), and fan deposits.
The Kozlu and Karadon Formations
The Kozlu Formation conformably overlies the Alacaagzi and is highly significant for its 19 coal seams, totaling 30–32 meters in thickness. It is composed of alternating conglomerate, sand, silt, mud, and coal. Finally, the Karadon Formation, which is lithologically similar to the Kozlu but with fewer coal seams, caps this sequence. The Paleozoic era concludes here with an angular unconformity lasting between 46.5 Ma and 194 Ma.
Localized Deposition
Following the major Paleozoic event, deposition resumed sporadically in the western portion of the basin. The Çakraz Formation spanned the upper Permian through the Lower Jurassic. This was followed by the Inalti Formation in the upper Jurassic, characterized by carbonates representative of a shallow passive margin setting, before being truncated by another unconformity.
Cretaceous Deposition
During the late Cretaceous, the Zonguldak basin was positioned at approximately 25° N. The region experienced subsidence driven by the formation of the Black Sea back arc, leading to a new period of deposition from the early Cretaceous through the Eocene.
The lithology of this era includes limestones, mudstones, siltstones, and dolomites. Key formations include:
- Zonguldak Formation (Albian, 105–100 Ma): Predominantly limestone with areas of dolomitization.
- Kilimli Formation (Albian, 105–112 Ma): Composed of sandstone and carbonaceous sandstone.
- Cemaller Formation: Sandy limestone that overlies the Kilimli after an unconformity of at least 6.5 Ma.
- Baþköy Formation: Siltstone and limestone overlying the Cemaller.
- Dinlence Formation: Consists of andesites and andesitic tuffites (volcanic ash rocks); it is potentially the same as the Yemislicay Formation.
- Alapý Formation: The uppermost layer, consisting of marls and limestones.
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Stratigraphic Summary
| Period/Era | Formation | Primary Lithology | Key Characteristics |
|---|---|---|---|
| Lower Ordovician | Soğuksu | Green shale, sandstone, conglomerate | 700–1100m thick |
| Upper Ordovician - Devonian | Findikli | Siliciclastic-carbonate | Shallowing shelf environment |
| Visean | Yilanli | Dolomitic limestone, shales | Start of coal-related sequences |
| Paleozoic | Alacaagzi | Shales, silts, sands, coal | Economic coal deposits; coastal facies |
| Paleozoic | Kozlu | Conglomerate, sand, silt, coal | 19 coal seams (30–32m total) |
| Permian - Lower Jurassic | Çakraz | Various | Localized western deposition |
| Albian | Zonguldak | Limestone, dolomite | Cretaceous subsidence phase |
| Cretaceous | Dinlence | Andesites, tuffites | Volcanic activity |
Frequently Asked Questions
Which formations in the Zonguldak basin contain economic coal?
The Alacaagzi Formation and the Kozlu Formation are the primary coal-bearing units. The Kozlu Formation is particularly notable for containing 19 coal seams with a combined thickness of 30–32 meters.
What caused the deposition during the Cretaceous period?
Deposition during the late Cretaceous was driven by subsidence resulting from the formation of the Black Sea back arc.
What is the significance of the Karadon Formation's top layer?
The top of the Karadon Formation is marked by an angular unconformity, representing a significant gap in the geological record lasting between 46.5 Ma and 194 Ma.
What environment does the Findikli Formation represent?
The Findikli Formation represents a mixed siliciclastic-carbonate shelf environment that became progressively shallower over time.
What are the characteristics of the Dinlence Formation?
The Dinlence Formation consists of andesites and andesitic tuffites, and it is suggested by some research to be the same as the Yemislicay Formation.